1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a control method therefor.
2. Description of the Related Art
In recent years an original image reading device that automatically feeds an original stacked on a document feeding device and reads images on front and back sides of the original concurrently in one pass through a conveyance path without turning the original upside down (e.g., see Japanese Laid-Open Patent Publication (Kokai) No. 2004-343409 (JP2004-343409A)) has been suggested. In such an original image reading device, a reading speed for a one-sided original on which an original image is formed on only one side is almost equal to that for a two-sided original on which original images are formed on both sides. Therefore, even if one-sided originals and two-sided originals are mixed in originals to be read, it is possible to read images at high speed without considering a difference between reading speeds of a one-sided original and a two-sided original.
On the other hand, in an image forming apparatus that prints a read image, it is necessary to have a pair of printing mechanisms for front and back sides independently in order to print images on both of front and back sides in one pass through a conveyance path. However, since the printing mechanism is complicated and expensive in an image forming apparatus by electrophotography printing, a plurality of printing mechanisms will enlarge the apparatus and increase a cost. Therefore, two-sided printing is performed by a single printing mechanism by printing a front side and a back side alternately.
Since in the image forming apparatus of such a configuration it is necessary to convey a recording sheet medium to an image forming unit again after performing processes such as printing on a front side of the recording sheet medium and a turnover of printed sides, a conveyance path for the recording sheet medium necessarily becomes longer. Therefore, when images are printed on the front and back sides of the recording sheet medium alternately, a waiting time from completion of printing on the front side to a start of printing on the back side becomes extremely long, which remarkably reduces productivity when a plurality of sheets are printed.
Then, a through pass two-sided method that minimizes a reduction of the productivity when performing a large amount of two-sided prints has been suggested. In this method, front sides of two or three recording sheet mediums are printed first, and then, the recording sheet mediums that are turned over to be printed on their back sides and new recording sheet mediums to be printed on their front sides are alternately printed. This technique is disclosed in U.S. Pat. No. 4,935,786.
Conventionally, an image forming apparatus, which has an original image reading device that automatically reads a two-sided original, which determines whether an original is a one-sided original or a two-sided original, and which switches between a one-sided printing and a two-sided printing automatically according to the determination result, has been suggested. This technique is disclosed in, for example, Japanese Laid-Open Patent Publication (Kokai) No. H09-190021 (JP H09-190021A), Japanese Laid-Open Patent Publication (Kokai) No. H10-268587 (JP H10-268587A), etc.
However, the above-mentioned conventional techniques have the following problems.
First, in the image forming apparatus performing a two-sided printing by the through pass two-sided method disclosed in the U.S. Pat. No. 4,935,786 etc., there is the following problem.
When copying a batch of originals in which one-sided originals and two-sided originals are mixed, an operator is necessary to set the one-sided printing or the two-sided printing to be performed beforehand according to the originals, which is complicated for the operator.
Furthermore, although the image forming apparatus in which it is possible to switch between the one-sided printing and the two-sided printing automatically as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H09-190021 (JP H09-190021A) and Japanese Laid-Open Patent Publication (Kokai) No. H10-268587 (JP H10-268587A) does not need the setting operation for the one-sided printing or the two-sided printing, there is the following problem.
When the two-sided printing is switched to the one-sided printing, the one-sided printing must be re-started after outputting all the recording sheet mediums in the apparatus in order to stack the printed sheet mediums onto a sheet eject tray in a correct page order. That's because it is necessary to eject the sheet mediums staying in the conveyance path along which the sheet medium is re-conveyed to the image forming unit after completing processes such as the printing process for the front side of the recording sheet medium and the turning over, during the two-sided printing. Therefore, a time lag occurs after finishing the two-sided printing once until when the one-sided printing becomes possible to start. Accordingly, when copying a batch of originals in which one-sided originals and two-sided originals are mixed, the one-sided printing and the two-sided printing are frequently switched at a small number of sheets, which causes a problem of reducing print productivity.